Three-Dimensional Decoupling Co-catalyst from. Photo-absorbing Semiconductor as a New Strategy. to Boost Photoelectrochemical Water Splitting

Transcription

1 Supporting Information for Three-Dimensional Decoupling Co-catalyst from Photo-absorbing Semiconductor as a New Strategy to Boost Photoelectrochemical Water Splitting He Lin,, Xia Long,*, Yiming An, Dan Zhou, and Shihe Yang*,, Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen , China. Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. * longxia@pkusz.edu.cn * yangsh@pkusz.edu.cn/chsyang@ust.hk 1

2 Experimental Procedures Chemicals and instruments Fluorine-doped Tin Oxide (FTO) coated glasses as substrates were purchased from PV Tech ( Fe(NO 3 ) 3 9H 2 O, NiCl 2 6H 2 O, AgNO 3, pyrrole, phytic acid, ethanol, were purchased from Sigma Aldrich without further treatment. Deionized water with a resistivity of Mohm cm was used in all reactions. All experimental results have been redone for more than three times to make sure the reproducibility. Preparation of TiO 2 Nanorod Array Films Firstly, the TiO 2 seeds were coated on FTO glass by spin coating TiO 2 sol followed by sintering at 550 C for 30 min. Then, the TiO 2 nanorod array films were synthesized on the seeded-fto substrates by using the hydrothermal method according to the previous work in our group. (J. Phys. Chem. Lett. 2014, 5, ) In a typical synthesis, 1 ml of titanium (IV) butoxide was added into 60 ml of an aqueous HCl solution (30 ml of concentrated HCl (36-38%) +30 ml of deionized water) under magnetic stirring. After 6 hours stirring, 10 ml of above solution was poured into a Teflon-lined stainless-steel autoclave (20 ml capacity) and a piece of seeded-fto (1.5 cm 3 cm) was immersed in the solution. The autoclave was sealed and heated to 200 o C for 5 hours in an oven. After cooling down to room temperature, the obtained products were washed with deionized water followed by absolute ethanol and finally annealed at 550 o C for 2 h in air. 2

3 Preparation of polymer coated TiO 2 Nanorod arrays photoanodes 30 μl pyrrole was diluted in 20 ml distilled water, and 0.01 M phytic dipotassium was dissolved in 10 ml DI water with 0.5 ml nitric acid. Then 1 ml pyrrole solution and ml phytic acid solution was mixed, then 200 μl mixed solution was drop added onto TiO 2 nanorod arrays by spin coating with the rate of 1000 rpm. Finally, the photoelectrodes were polymerized under the illumination with 254 nm UV light for 10 h. Preparation of TMO-polymer coated TiO 2 Nanorod arrays photoanodes 30 μl pyrrole was diluted in 20 ml distilled water, and 0.01 M phytic dipotassium was dissolved in 10 ml DI water with 0.5 ml nitric acid. Then 160 μl of saturated iron/nickel/silver nitric aqueous solution was added into the mixed polymer solution, then 360 μl mixed metal nitric and polymer solution was quickly dropped onto TiO 2 nanorod arrays by spin coating with the spin rate of 1000 rpm, then illuminated with 254 nm UV light for 10 h. Preparation of FeO x /TiO 2 and CP/TiO 2 (dissolving FeO x ) FeO x /TiO 2 photoelectrodes were prepared by annealing the FeO x -CP/TiO 2 photoelectrodes in air at 550 C for 3 hours. CP/TiO 2 (dissolving FeO x ) photoelectrodes were prepared by immersing the FeO x -CP/TiO 2 photoelectrodes in 0.01 M HCl for 3 days. Preparation of TiO 2 nanoparticle thin film (planar structure, pl-tio 2 )-based photoanodes The TiO 2 paste (Dyesol Australia PtyLtd) was diluted with 2.5 times of ethanol by weight and was stirred for 30 min. Then the diluted TiO 2 paste was spin-coated on FTO substrate at

4 rpm for 30 s. After spinning, the thin film was dried at 120 for 5 min and calcination at 550 for 2 h. The CP/pl-TiO 2, and CP-FeO x /pl-tio 2 photoanodes were prepared by the same decoupling method with same conditions for preparing CP/TiO 2 (nanorod ararys) and CP-FeO x /TiO 2 (nanorod arrays), except that the photoabsorber semiconductor were TiO 2 nanoparticle thin film rather than TiO 2 nanorod arrays. Preparation of WO 3 nanosheet arrays-based photoanodes WO 3 nanosheet arrays were prepared via a facile hydrothermal growth method similar to previously reported work (Nanoscale, 2015, 7, ). Typically, a seed thin layer was spin coated onto the FTO substrates prior to the hydrothermal process. The seed solution was prepared by dissolving 1.25 g H 2 WO 4 powder into 10 ml 50% H 2 O 2 at 70 o C. After spin coating, the FTO glasses were annealed at 550 o C for 2 h in air. Then, 1.65g of Na 2 WO 4 2H 2 O was dissolved in 37.5 ml of water with stirring, and the 3 M HCl was added dropwise until the ph of solution decreased to the value of 2. After 2 hours stirring, 18 ml of above solution was poured into a Teflon-lined stainless-steel autoclave (25 ml capacity) and a piece of seeded-fto (1.5 cm 3 cm) was immersed in the solution. The autoclave was sealed and heated to 180 o C for 6 hours in an oven. After cooling down to room temperature, the obtained products were washed with deionized water followed by absolute ethanol and finally annealed at 550 o C for 2 h in air. The CP/WO 3, and CP-FeO x /WO 3 photoanodes were prepared by the same decoupling method with same conditions for preparing CP/TiO 2 (nanorod ararys) and CP-FeO x /TiO 2 (nanorod arrays), except that the photoabsorber semiconductor were WO 3 nanosheet arrays rather than TiO 2 nanorod arrays. 4

5 Photoelectrochemical measurements The solar light simulator (Newport solar simulator, model number 6255, 150 W Xe lamp, AM 1.5G global filter) was calibrated to 1 sun (100 mw cm -2 ) using a silicon reference solar cell equipped with KG-5 filter. The photoelectrochemical properties were measured by an IM6x electrochemical workstation (ZAHNER-Elektrik GmbH & Co., KG, Germany) in a standard three-electrode system, and 0.5 M phosphate buffer with ph=7 was used as the electrolyte. The sulfite oxidation was conducted in a 0.5 M phosphate buffer solution containing 1 M sodium sulfite (ph = 7) under one sun illumination. Photocurrent vs. voltage (J-V) characteristics were recorded by scanning potential from -0.6 to 1.0 V (vs. Ag/AgCl) with a scan rate of 10 mv/s. The measured potentials vs. Ag/AgCl reference electrode were converted to the reversible hydrogen electrode (RHE) scale using the relationship of E(RHE)=E(Ag/AgCl) *pH + E 0 (Ag/AgCl), where E(Ag/AgCl) is the experimentally measured potential and E 0 (Ag/AgCl) = 0.209V at 25. The incident photon to current efficiency (IPCE) was determined using monochromatic light irradiation with Zahnar-Elektrik. IPCE was measured at 1.23 V (vs. RHE) in phosphate buffer (PH=7) solution using the same three-electrode setup described above for photocurrent measurements. IPCE was calculated as follow: IPCE (%) = ma 1240 I cm2 Plight mw 100(%) cm2 λ nm Where I is the measured photocurrent density at specific wavelength, λ is the wavelength of incident light, and Plight is the measured light power density at that wavelength. 5

6 Supposing 100% Faradaic efficiency, the applied bias photo-to-current efficiency (ABPE) was calculated from the J-V curve by following equation: ABPE (%) = I ma cm Vbias V Plight mw cm2 100(%) Where I is the photocurrent density, Vbias is the applied potential, Plight is the incident illumination power density (100 mw/cm 2 ). The electrochemical impedance spectroscopy (EIS) Nyquist plots were obtained at 0.5 V (vs. RHE) (the potential when ABPE reached the peak value) with small AC amplitude of 10 mv in the frequency range of 1 to 4M Hz. The measured spectra were fitter with Zview. The surface charge separation sufficiency ---η(surface) was calculated using the equation of η surface % = J PEC J sulphite oxidation 100 % Mott-Schottky plots were collected by using frequency of 1 khz, an Ac amplitude of 5 mv. Measurements were taken on a CHI 760E electrochemical analyzer in the dark with three electrode system in 0.5 M potassium phosphate buffer solution (ph=7). The carrier densities can be calculated from Mott-Schottky plots use the equation: N d = 2 qεε 0 d 1 C 2 dv 1 where N d is the carrier density, ε is the rutile TiO 2 dielectric constant (ε = 86) (J. Am. Chem. Soc. 2013, 135, ), ε 0 is the vacuum permittivity, and the quantities q is electronic charge. 6

7 Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) complex plane plots for the as synthesized photoelectrodes were illuminated with a blue light emitting diodes (l max = 370 nm) at a light intensity of 10 W/m 2. The electron transport coefficients (D n ) calculated from the IMPS measurements according to the following equation: D n = 2πf min L 2 /2.35 where f min is the frequency at the minimum in the complex plane IMPS plot, and L is the film thickness (700nm). The electron lifetime t d can be obtained from the IMVS plot by the formula: t d = 1/2πf min where f min represents the frequency at the minimum point on the IMVS complex plane plot. The produced H 2 and O 2 were collected and analyzed on gas chromatograph (Shimadzu, GC- 2014, Japan) equipped with a thermal conductivity detector (TCD). In detail, 1 ml of gas product was taken from the reactor at given intervals (1 h) during the CP test and used for gas component analysis. Products were calibrated with a standard gas and were determined by the retention time. The carrier gas used in the GC-2014C was high purity nitrogen. Morphological and structure characterizations Scanning electrochemical microscopy (SEM) and energy dispersive X ray spectroscopy (EDS) were characterized by JEOL JSM Transmission electron microscopy (TEM) samples were prepared by sonicating the photoelectrode in ethanol and then the suspensions were drop dried onto copper grids and were measured by JEOL JEM

8 Other characterizations X-ray diffraction (XRD) were measured by a multi-purpose XRD diffractometer of PANanalytical, model Empyrean. X-ray photoelectron spectroscopy (XPS) spectra were collected on Physical Electronics, model PHI Micro FTIR spectra were characterized by a FTIR spectrometer, model Vertex 70 Hyperion Ultraviolet-visible (UV-vis) diffusive reflectance and absorbance spectroscopy were measured by using an UV-vis spectrophotometer (PerkinElmer, model Lambda 20). 8

9 Results and Discussion A B Scheme S1. Schematic illustration of light absorption on the surface of (A) traditional photoelectrode with cocatalyst nanoparticles and (B) FeO x -CP/TiO 2 photoanode prepared in this work with decoupled photoelectrode and cocatalyst nanoparticles dispersed in the pore-spanning CP. 0.8 C -2 (cm -4 F -2 )* CP Potential (V vs RHE) Figure S1. Mott-Schottky plots of bare CP. The negative slope indicates that the as-prepared CP is p-type semiconductor. 9

10 Figure S2. SEM images of TiO2 nanorod arrays on FTO. (A, B) top view, (C) side view. Figure S3. TEM images of CP-FeOx/TiO2 photoanodes with various magnifications. (A-C) TEM images showing the cocatalyst nanoparticles on the TiO2 nanorods; (D-F) thin amorphous layer of CP matrix film on TiO2 nanorods. The arrows in B-C indicate FeOx cocatalyst nanoparticles, and the red dotted lines in D-F indicates the amorphous CP matrix layers either on the surface of TiO2 nanorods or formed in between adjacent TiO2 nanorods. 10

11 Intensity (a,u.) FTO TiO2 * CP-FeOx/TiO2 # * Q (deg.) Figure S4. XRD spectra of the as-synthesized photoanodes of bare FTO (blue curve), TiO 2 nanorods array on FTO (red curve) and CP-FeO x /TiO 2 on FTO (black curve). * represents Fe 3 O 4, and # represents Fe 2 O 3. Intensity (a.u.) Fe(III) sat. Fe 2p1/2 Fe 2p3/2 Fe(III) Fe(II) Fe(II) sat Binding Energy (ev) Figure S5. High resolution XPS spectrum of CP-FeO x /TiO 2 in Fe 2p spin-orbital region. The magenta curves, blue curves and yellow curves represent Fe (III), Fe (II) and satellites, respectively. 11

12 CP-FeOx/TiO2 CP/TiO2 FTO TiO2 0.6 T Wave number (cm -1 ) Figure S6. Micro FTIR spectra of bare FTO (magenta curve), TiO 2 (green curve), CP/TiO 2 (red curve) and CP-FeO x /TiO 2 (black curve) photoanodes. The blue arrows indicate characteristic bands of CP. Figure S7. SEM images of CP-Ag oxides on TiO 2 nanorod arrays with different magnifications. 12

13 Figure S8. SEM images of CP-Ni oxides on TiO2 nanorod arrays with different magnifications. Figure S9. SEM images of CP/TiO2 nanorod arrays on FTO. (A, B) top view images and (C, D) side view images with different magnifications. 13

14 Figure S10. SEM images of CP/TiO 2 photoanode made by dissolving FeO x cocatalyst nanoparticles of CP-FeO x /TiO 2 with different magnifications. The red arrows in B indicate the retainment of crosslinked CP matrix that links adjacent TiO 2 nanorods. Figure S11. SEM images of FeO x /TiO 2 photoanode prepared by removing CP matrix of CP- FeO x /TiO 2 with different magnifications. (A) SEM image in low magnification with scale bar of 1 µm, (B) SEM image in high magnification with scale bar of 100 nm. From the high-resolution SEM image (Figure S11B), one can see that FeO x cocatalyst nanoparticles are firmly attached on the TiO 2 nanorod arrays, which inhibit the light absorption. 14

15 1.5 CP/TiO2 CP-FeOx/TiO2 TiO2 1.2 j (ma/cm 2 ) E (V vs RHE) Figure S12. J-V curves of the as-synthesized photoanode under illumination. Red curve: CP- FeO x /TiO 2, black curve: CP/TiO 2, and blue curve: bare TiO FeO x /TiO 2 -dark FeO x /TiO 2 -light j(ma/cm 2 ) E (V vs RHE) Figure S13. J-V curves of the photoelectrodes of FeO x /TiO 2. Black curve: dark, red curve: light. 15

16 ( 1 2 /2 ) ( ( ). 4 Figure S14. J-V curves of CP/TiO 2 photoelectrodes made by dissolving FeO x cocatalyst nanoparticles in 0.01M HCl with different time. 1.0 j (ma/cm 2 ) light dark E(V vs RHE) Figure S15. J-V curves of the as-synthesized CP on FTO. Black curve: light, red curve: dark. Bare CP matrix showed negligible response on illumination (Figure S15), suggesting CP matrix mainly works as an electron blocking layer to facilitate the charge separation and hole transport. 16

17 j(ma/cm 2 ) CP/TiO 2 (dissolving FeO x )-Drak CP/TiO 2 (dissolving FeO x )-Light CP/TiO 2 -Drak CP/TiO 2 -Light E (V vs RHE) Figure S16. J-V curves of the photoelectrodes of CP/TiO 2, and CP/TiO 2 (after dissolving FeO x, with crosslinked microstructure). The photocurrent density (Figure S16) of the CP/TiO 2 (made by dissolving FeO x for 3 days) was higher than that of CP/TiO 2 prepared simply by photopolymerization process (CP matrix only covered on the surface of TiO 2 nanorods, Figure S9), highlighting the importance of crosslinking and nanostructuring of the CP phase in improving the PEC performance. Although the removal of FeO x might be incomplete (yet it is below detection limit) in the photoelectrode of CP/TiO 2, the morphology and location of the CP matrix are definitely important factors for the promoted PEC water splitting performance. In other words, the intergrown and intertwined CP nanomatrix in the inner space of the TiO 2 nanorods should be more beneficial to the charge separation and transport than the CP matrix without the network structure. 17

18 Figure S17. SEM images of WO3 nanosheet arrays-based photoanodes. (A, B) WO3, (C, D) CP/WO3, (E, F) CP-FeOx/WO3 images with different magnifications. The red arrows in E indicate CP-FeOx matrix that linked adjacent WO3 nanosheets. Figure S18. SEM images of TiO2 nanoparticle film (pl-tio2)-based photonades on FTO. (A, B) TiO2 nanoparticle thin film, (C, D) CP/pl-TiO2 and (E, F) CP-FeOx/pl-TiO2. The inset in A is the side view SEM image of the TiO2 nanoparticle thin film on FTO, confirming its planar structure with thickness of around 70 nm. 18

19 2.0 WO CP/WO 3 CP-FeO x /WO 3 j (ma/cm 2 ) E(V vs RHE) Figure S19. J-V curves of the as-synthesized photoanode under illumination. Black curve: WO 3, red curve: CP/WO 3, and blue curve: CP-FeO x /WO 3. Figure S20. Digital images showing the bubble generation behavior difference on (A) CP-FeO x / pl-tio 2 and (B) CP-FeO x /TiO 2. 19

20 160 j (μa/cm 2 ) TiO 2 CP/TiO 2 CP-FeO x /TiO E (V vs RHE) Figure S21. J-V curves of the as-synthesized photoanode under illumination. Black curve: bare TiO 2 nanoparticle thin film, red curve: CP/TiO 2 nanoparticle thin film, and blue curve: CP- FeO x /TiO 2 nanoparticle thin film. Figure S22. Mott-Schottky plots of (A) bare TiO 2, (B) CP/TiO 2 (after dissolving FeO x, with crosslinked microstructure), (C) CP/TiO 2, (D) FeO x /TiO 2, and (E) CP-FeO x /TiO 2. 20

21 A B C j (ma/cm 2 ) sulfite oxidation PEC j (ma/cm 2 ) sulfite oxidation PEC j (ma/cm 2 ) sulfite oxidation PEC E (V vs RHE) E (V vs RHE) E (V vs RHE) Figure S23. J-V curves of as-synthesized photoanodes for water splitting (red curves) and sulfite oxidation (black curves). (A) TiO 2, (B) CP/TiO 2, (C) CP-FeO x /TiO 2. The sulfite oxidation was conducted in a 0.5 M phosphate buffer solution containing 1 M sodium sulfite (ph = 7) under one sun illumination. The J-V curves were corrected by subtracting the background to avoid the influence of packaging problems. Figure S24. UV-Vis diffusive reflectance and absorbance spectra of CP/TiO 2, CP-FeO x /TiO 2, and TiO 2 photoanodes. 21

22 Figure S25. Intensity-modulated photocurrent spectroscopy (IMPS) plots of (A) bare TiO 2, (B) CP/TiO 2 (after dissolving FeO x ), (C) CP-TiO 2, (D) CP-FeO x /TiO 2. Figure S26. Intensity-modulated photovoltage spectroscopy (IMVS) plots of (A) bare TiO 2, (B) CP/TiO 2 (after dissolving FeO x ), (C) CP-TiO 2, and (D) CP-FeO x /TiO 2. 22

23 E (V vs RHE) t (min) Figure S27. J-V curve collected on CP-FeO x /TiO 2 photoelectrode at the fixed current density of 10 ma/cm 2. Figure S28. XPS spectrum of CP-FeO x /TiO 2 after stability test. 23

24 Figure S29. Gas chromatogram (GC) of PEC water splitting on CP-FeO x /TiO 2 photoelectrode. Table S1. Summary of the calculated flat band potentials and carrier densities of as-prepared photoelectrodes from Mott-Schottky plots. TiO 2 CP/TiO 2 (Dissolving FeO x ) CP/TiO 2 FeO x /TiO 2 CP-FeO x /TiO 2 E FB (V vs. RHE) (±0.02) (±0.02) (±0.03) (±0.02) (±0.02) N d (cm -3 ) 1.95* * * 1.13* 1.11* 24

25 Table S2. Summary of the calculated values of series resistance (R s ) and charge transfer resistance (R ct ) of the as-prepared photoelectrodes, by fitting the Nyquist plots with equivalent circuit (inset in Figure 4A). TiO 2 CP/TiO 2 FeO x /TiO 2 CP-FeO x /TiO 2 R s (ohm) R ct (ohm)